Explicitly assert count on returned data size

Co-authored-by: Weston Ruter <[email protected]>

Author: aduth

tests/phpunit/tests/rest-api/rest-themes-controller.php

@@ -239,7 +239,7 @@ public function test_get_items_active() {
 			'theme_supports',
 		);
 		$this->assertIsArray( $data );
-		$this->assertNotEmpty( $data );
+		$this->assertCount( 1, $data );
 		$this->assertSameSets( $fields, array_keys( $data[0] ) );
 
 		$this->assertEquals( array( 'rest-api' ), wp_list_pluck( $data, 'stylesheet' ) );